The present invention relates to a thermosetting resin composition, a process for the preparation of this composition, and a cured product obtained by heating and curing this composition.
Thermosetting resins are now broadly used for not only electric parts and electronic but also parts of various machines such as automobiles and business machines. In parts of machines, application conditions have become severe, and development of materials excellent in the heat resistance, mechanical properties and electric properties is eagerly desired.
As the material excellent in the heat resistance, aromatic polyimides and aromatic polyamides are known and they are used as coating materials for enameled wires. Although these materials are excellent in the heat resistance, mechanical properties and electric properties, the curing temperature is as high as about 300.degree. C. and since curing is accomplished by dehydrating ring closure reaction, water is formed as a by-product and the molding operation becomes troublesome.
We made researches with a view to developing molding materials (inclusive casting materials; casting materials will be included in molding materials hereinafter) having properties suitable for machine parts, and found that a fluid resin composition comprising polymaleimide is satisfactory. A bismaleimide type resin composition similar to this polymaleimide resin composition was already proposed in Japanese Patent Publication No. 13676/78. This known composition is characterized in that the composition comprises an allyl ester of a polyvalent carboxylic acid and a mono- or bis-maleimide. Most of maleimide compounds are solid at room temperature and in order to obtain homogeneous fluid compositions comprising maleimide compounds, it is necessary that the maleimide compounds should be dissolved at temperatures substantially higher than the melting points thereof in reactive solvents (liquid cross linking agent; the same will apply hereinafter). However, as the solution temperatures are lowered, the maleimide compounds tend to precipitate in the form of crystals from the solutions. To our surprise, it was found that a polymaleimide has a good compatibility with a reactive solvent and a solution of a polymaleimide in a reactive solvent is homogeneous and stable even at low temperatures and therefore, impregnation or casting can easily be accomplished by using this solution without particular heating. It also was found that the resulting cured product is excellent in the resistance to thermal degradation.